The present invention relates to an electrical circuit and method for providing drive signals for a flux-gate magnetic sensor used in connection with an automotive or vehicle compass.
Typically, vehicle mounted compasses have been mechanical types with rotating compass cards mounted in a viscous damping media. An electrical compass has been described in a 1980 SAE paper entitled Magnetic Field Sensor and Its Application to Automobiles by Hisatsugu Itoh, SAE/SP-80/458. This compass employs a flux-gate sensor and solid state circuits. U.S. Pat. No. 4,546,551 entitled ELECTRICAL CONTROL SYSTEM and assigned to the present assignee discloses an automotive compass in which a flux-gate sensor is employed and which employs digital bandpass filtration to process signals from the sensor. The filter and compass system is also disclosed in U.S. Pat. No. 4,424,631, assigned to the present assignee.
A flux-gate sensor, as is well known, is driven by a fundamental frequency, and information is obtained from the amplitude and polarity changes of the second harmonic frequency signals obtained from the sensor. In order to precisely detect the second harmonic signals, it is necessary to employ synchronous phase detectors to assure the information from the two channels is synchronized to provide the desired vectored output information corresponding to vehicle heading. In the past, either analog or digital filtering has been required to provide a sinusoidal second harmonic signal which can be processed to supply the necessary information. The digital filtration system has proven successful in providing the desired information, however, only with the addition of significant circuit expense for the digital filter itself as well as associated circuit elements.